1. Field of the Invention
The present invention relates to ballpoint pens. More specifically, the present invention is directed to a so-called pipe type ballpoint pen having a ballpoint pen tip at the front end thereof, the ballpoint pen tip rotatably accommodating a ball at the front end of a pipe made of metal. The invention is particularly related to a pipe type ballpoint pen adapted for writing thin characters.
2. Description of the Related Art
A well known conventional ballpoint pen is so designed that a ballpoint pen tip which rotatably accommodates a ball at the front end of a pipe made of metal is force fitted into a I5 holder made of a synthetic resin. (For example, U.S. Pat. No. 4,311,403).
However, the ballpoint pen is usually used while inclined with respect to a writing surface. Therefore, a bending moment is applied to the front pipe portion at all times. When a large writing pressure (more specifically, a vertical load of about 1.0 kg or more with respect to the writing surface at a writing angle of 60 to 75.degree.) is applied to the ballpoint pen tip in the conventional ballpoint pen, the writing pressure, of course, varying from one user to another, the front pipe portion is bent at the joint thereof. Particularly, in a pipe type ballpoint pen having a ball receiving seat formed by utilizing deformation of a metallic material by pressing, it is not practical to improve bending strength by increasing the thickness of the front pipe portion since it is difficult to manufacture a ballpoint pen having a very large thickness of the front pipe portion.
Further, the conventional ballpoint pen is usually such that the outer diameter of the front pipe portion and the outer diameter of the ball accommodated at the front end thereof are large (more specifically, the outer diameter of the front pipe portion is set to 0.8 mm or more and the outer diameter of the ball at the front end is set to 0.5 mm or more), which produces thick handwritten characters, thus making the ballpoint pen unsuitable for jotting down thin characters into small spaces in a pocketbook or the like. If the outer diameter of the front pipe portion and that of the ball at the front end are reduced in order to obtain a ballpoint pen suitable for producing thin handwritten characters, chances of the front pipe portion being bent with even a slight writing pressure increase.
If the front pipe portion has a thin wall thickness by having a ball receiving seat formed by inward deformation thereof (more specifically, the thickness of the front pipe portion is set to 0.15 mm or less), or if the front pipe portion has a small diameter to obtain a ballpoint pen tip dedicated to writing thin characters (more specifically, the outer diameter of the front pipe portion is set to 0.75 mm or less), or if the front pipe portion is hollow to allow ink to flow therethrough directly while having no ink inducing member therein, then the front pipe portion shakes with additional flexion of the pipe itself at the time of writing, which not only gives a sense of unstable writing to the user, but also increases the possibility of the front pipe portion being bent.
In the conventional ballpoint pen, the fixing strength between the holder and the pipe tends to be reduced due to secular change or the like if the holder is made of a synthetic resin. This causes the pipe greatly to shake at the time of writing, thereby giving the user a sense of unstable writing. Additionally, in the case of the holder made of a synthetic resin, large writing pressure and falling impact onto the floor applied to the ballpoint pen tip would cause the ballpoint pen tip to be pushed rearward and thereby be collapsed into the holder by an excessive force, which increases the likelihood of the user not being able to write with the ballpoint pen.
Still further, Japanese Unexamined Utility Model Publication No. 62-5978 and Japanese Examined Utility Model Publication No. 2-5994 disclose ballpoint pens in which a seal member made of an elastic member is arranged within a cap to seal the ballpoint pen tip of the ballpoint pen.
However, when the outer diameter of the front pipe portion and that of the ball at the front end are reduced in order to obtain a ballpoint pen suitable for writing thin characters, the seal member disclosed in the aforementioned Japanese Unexamined Utility Model Publication No. 62-5978 has recess portions so thin that the recess is broken due to excessive stress concentration caused by repeated abutment of the sharp ballpoint pen tip of the ballpoint pen thereagainst.
In addition, the seal member disclosed in Japanese Examined Utility Model Publication No. 2-5994 is of such a construction that almost all deformation of the ballpoint pen tip of the ballpoint pen due to the abutment is accommodated by the thin joining portion thereof. In addition, the joining portion of this seal member is short in the axial direction, and instantly produces resiliency due to tension upon abutment of the ballpoint pen tip against the recess. As a result of this construction, if the length of movement of the joining portion after the abutment of the ballpoint pen tip against the recess (the amount of pushing) is increased due to variations in the radial direction or in order to ensure sufficient sealing of the ballpoint pen tip, strong resiliency is applied to bias the ballpoint pen tip. Hence, when the outer diameter of the front pipe portion and that of the ball at the front are similarly reduced in order to obtain a ballpoint pen suitable for writing thin characters, there exists the possibility of the ballpoint pen tip sticking into the recess.
In other words, the conventional seal member is not sufficiently given consideration to the possibility of the seal member which is broken or stuck by the front pipe portion, since the outer diameter of the front pipe portion of the ballpoint pen is comparatively large (0.8 mm or more). That is, a seal member optimal for a ballpoint pen tip whose front pipe portion has such a small diameter as to allow thin characters to be written has not yet been disclosed.